Alamouti-type space frequency block coding (SFBC) has been accepted as a diversity scheme in the third generation partnership project (3GPP) long term evolution (LTE) for two transmit antenna devices due to its excellent performance and simple decoding. SFBC may be extended to four transmit antenna devices by combining SFBC with frequency switch transmit diversity (FSTD).
Alamouti scheme of space time codes (STC) involves transmission of multiple redundant copies of data. In SFBC, the data stream to be transmitted is encoded in blocks and distributed among spaced antennas and across multiple subcarriers. While it is necessary to have multiple transmit antennas, it is not necessary to have multiple receive antennas, although to do so improves performance.
Although SFBC and its variants such as SFBC/FSTD achieve good performance in interference free environment, it imposes undesirable structure to intercell interference (ICI). In a conventional two-transmit antenna system, a first Node B transmits SFBC encoded data to a first user equipment (UE). In the meantime, a neighboring Node B (a second Node B) transmits SFBC encoded data to a second UE over the same frequency band. The transmission from the second Node B to the second UE works as an interference to the first UE.
The SFBC encoder in the first Node B encodes two incoming data signal s1 and s2 according to well known Alamouti scheme as follows:
                              S          =                      (                                                                                s                    1                                                                                        -                                          s                      2                      *                                                                                                                                        s                    2                                                                                        s                    1                    *                                                                        )                          ;                            Equation        ⁢                                  ⁢                  (          1          )                    where a row index is a spatial index and a column index is a frequency index.
Likewise, the SFBC-encoded data from the second Node B, which works as an interference to the first UE, can be expressed as follows:
                    I        =                              (                                                                                i                    1                                                                                        -                                          i                      2                      *                                                                                                                                        i                    2                                                                                        i                    1                    *                                                                        )                    .                                    Equation        ⁢                                  ⁢                  (          2          )                    
Assuming channel responses are equal for two adjacent frequency subcarriers, and a channel coefficient matrix H associated with the first UE and a channel coefficient matrix G associated with the second UE are as follows:H=(h1 h2);  Equation (3)andG=(g1 g2).  Equation (4)
It is assumed that a single receive antenna is used at the UE in the equations above. However, it can be extended to multiple antennas. The received signal at two subcarriers at the first UE can be written as follows:
                              (                                                                      y                  1                                                                                                      y                  2                  *                                                              )                =                                            (                                                                                          h                      1                                                                                                  h                      2                                                                                                                                  h                      2                      *                                                                                                  -                                              h                        1                        *                                                                                                        )                        ⁢                          (                                                                                          s                      1                                                                                                                                  s                      2                                                                                  )                                +                                    (                                                                                          g                      1                                                                                                  g                      2                                                                                                                                  g                      2                      *                                                                                                  -                                              g                        1                        *                                                                                                        )                        ⁢                          (                                                                                          i                      1                                                                                                                                  i                      2                                                                                  )                                +                                    (                                                                                          n                      1                                                                                                                                  n                      2                                                                                  )                        .                                              Equation        ⁢                                  ⁢                  (          5          )                    
The second term in Equation (5) is interference term. The interference caused by i1 is spanned by (g1 g2), and interference caused by i2 is spanned by (g*2 −g*1). Since (g1 g2) and (g*2 −g*1) are orthogonal, the interference can not be cancelled.